1. Field of Art
This invention relates generally to improvements in internal combustion engine systems. More particularly, this invention relates to an internal combustion (“IC”) engine, and method of operation thereof, having combustion characteristics which combine certain favorable attributes of both typical stoichiometric combustion and “lean burn” combustion cycles, while at the same time having reduced exhaust emissions.
2. Related Art
The IC engine is used in innumerable applications today. Such engines combine relatively high power with relatively small size, in terms of power output/unit weight of the power source. This high power to weight ratio permits use of IC engines where space is a premium—in transportation sources such as automobiles, for example. Innumerable other installations use IC engines, as the driver for electric power generation, pumps, hydraulic systems and the like.
IC engines use a variety of fuel sources. Liquid fuels such as gasoline and diesel are perhaps the most commonly used fuels in IC engines in transportation, but some automobiles and many stationary IC engine installations use fuel which is in a gaseous form when supplied to the engine. Examples used in automobiles are propane-fueled systems, where the propane is in a liquid phase in the fuel tank, then is dropped to a lower pressure to vaporize it before supplying it to the engine. Stationary IC engines often run off of natural gas, predominantly methane from natural gas wells. Other sources of natural gas include waste material digesters and landfills. Obviously, use of natural gas from such sources is a very efficient and environmentally friendly process.
However, for all their positive attributes, a drawback of IC engines is that all produce by-products of combustion, which are found in the exhaust gas stream emanating from the engine, and which contribute to air pollution. While in a perfect combustion cycle, the by-products would be only carbon dioxide and water, in actual application a perfect combustion cycle cannot be attained. A goal is therefore the reduction of the undesirable by-products of combustion, and as a result reduce their contribution to air pollution. This goal is mandatory to some extent, as governmental entities implement and enforce increasingly strict air pollution controls.
IC Engine Ignition Mechanisms
IC engines can be grouped into (1) spark ignition engines, which rely on an induced spark (generally from a spark plug in the combustion chamber) to initiate the burning of the fuel/air mixture; and (2) compression ignition engines (commonly referred to as diesel engines) which operate at much higher compression ratios, and utilize the corresponding high temperature generated from the higher compression to auto-ignite the fuel/air mixture.
Primary Exhaust Emission Components
The three primary pollutant emission components in a spark ignition engine are CO (carbon monoxide), HC (hydrocarbon, also referred to as volatile organic compounds or VOCs, basically unburned hydrocarbon fuel), and NOx (nitrogen oxides of different formulae, which are a chief contributor to smog formation). In addition to those emissions, a compression ignition engine (e.g., a diesel engine) also emits PM (particulate matter, the black smoke typical of a diesel). The two ignition mechanisms typically exhibit some differences in emissions in real-life applications. For example, both CO and NOx emissions are lower with CI engines.
Stoichiometric v. Lean Burn Combustion Cycles
The mass ratio of air to fuel is an important consideration not only for operating efficiency but also emissions reasons. Stoichiometric engines, or more properly stoichiometric combustion processes, use approximately the chemically correct or “exact” amount of air/fuel ratio for combustion. Prior art engines operating under stoichiometric combustion cycles must utilize a relatively low compression ratio to prevent uncontrolled combustion or burn rate or “knock.” Relatively low compression ratios translate into relatively low fuel conversion efficiency. Typical compression ratios (defined as maximum cylinder volume/minimum cylinder volume) for SI engines in general are between 8 and 12, although depending upon many variables, compression ratios higher than this range may be possible (e.g., to around 16).
Another combustion cycle is the “lean burn” cycle, which has an excess of air in the air/fuel mixture, i.e. more air than is necessary for combustion. In comparison to a stoichiometric cycle, engines operating with a lean burn cycle typically employ a higher compression ratio, and the higher compression ratio combined with the fuel-lean air/fuel ratio is capable of yielding increased fuel conversion efficiency (translated into useful work per unit fuel used, such as miles per gallon). The higher compression ratios of lean burn cycles are possible in part because the combustion process is slower, with the excess air acting as an energy or heat sink to absorb some of the heat of combustion. Additional benefits of a lean burn cycle are usually reduced CO emissions, because there is plenty of oxygen to combine with CO to form CO2; and reduced HC emissions because the excess air (oxygen) tends to yield more complete combustion. Those are emission-related “pluses” from a lean burn cycle.
NOx emissions can decrease at very high air/fuel ratios, but in practice lean burn engines used for automotive applications operate at ratios not far above stoichiometric ratios, so this effect is not really seen. However, certain stationary power applications use very high air/fuel mass ratios, as high as λ=2, and at such high ratios NOx emissions are low. A tradeoff occurs at these high ratios, as HC emissions tend to increase due to an increased quench effect and possibility of misfiring.
The higher compression ratio in a lean burn engine usually generates higher combustion temperatures. These higher combustion temperatures tend to result in higher NOx production. In addition, another disadvantage of the lean burn cycle is the difficulty in treatment of NOx emissions, due to the presence of excess oxygen in the exhaust gas stream (the high affinity of the O2 makes it difficult to convert NOx into N2 and CO2). Typical non-selective catalytic reduction (“NSCR”) systems, common in automotive engine applications, are of little utility. So while the lean burn technology is generally more efficient, and may generate lesser CO and HC emissions, NOx emissions tend to be higher (all of this being in relation to a stoichiometric combustion cycle).
Catalytic Processing of Exhaust
In many engines (whether stoichiometric or lean burn), a “three-way” catalytic converter is used to lower CO, HC and NOx emissions by passing the exhaust gases through the converter. Catalytic converters used with Si engines comprise a bed of active catalytic material (usually in a metal casing) through which exhaust gases are flowed. Various bed arrangements are well known in the art (beads, porous elements, etc.). The catalysts themselves are of different types to effect different emission controls. Some catalytic converters are of a two-bed design. One bed contains oxidation catalysts oxidize CO and HC to CO2 and water, and typically comprise various noble metals (such as mixtures of platinum and palladium) well known in the art. Another bed contains other catalytic agents usually used for NOx reduction (well known in the art, such as base metal catalysts), which is preferably operating in a low-oxygen environment. IC engine operation at or near stoichiometric permits use of a single-bed, “three way” catalytic converter (also called a non-selective catalytic reduction or NSCR system), which both oxidizes CO and HC and reduces NOx. Three way catalysts are well known in the art, and are common in automotive applications. Operation of such three-way catalysts is enhanced with oxygen sensing in the engine exhaust to sense whether the engine is operating fuel rich or lean, and to adjust the air/fuel mixture to the engine accordingly to maintain nearstoichiometric operation.
As a result, while not as efficient as lean burn engines, stoichiometric burn engines have the advantage that an NSCR system can be used for high conversion rates of NOx, CO, and HC. In order to keep exhaust emissions low, the engine has to keep air/fuel ratio fairly constant around stoichiometric (the air/fuel ratio is commonly known as lambda, “λ,” and a stoichiometric ratio has λ=1; the “lambda window” refers to the range of lambda values slightly above and below 1). To achieve λ≅1, a closed loop air fuel ratio control system is used, incorporating an oxygen sensor placed in the exhaust gas stream to sense the presence of excess oxygen, and in response to a signal from the oxygen sensor a controller increases/decreases fuel delivery. The smaller the lambda window the better and more efficient the catalytic process. This process is a well proven technology for automotive systems using liquid fuels, in particular gasoline.
However, the use of gaseous fuels such as natural gas, digester gas, or landfill gas presents problems with this type of emission control system. Exhaust gas resulting from a combustion process burning gaseous fuel contains a higher concentration of hydrogen and light hydrocarbons compared to liquid fuels such as gasoline. These components lead to interference with “automotive type” oxygen sensors. The result is a low efficiency in pollutant conversion in the aftertreatment system.
A desirable goal is to have an IC engine system with the desirable characteristics of both stoichiometric and lean burn combustion cycles—relatively high efficiency, relatively low amounts of undesirable exhaust emissions, and efficient catalytic processing of such exhaust emissions as are generated. In addition, the IC engine system must be adapted to the use of gaseous hydrocarbon fuels.